This invention relates in general to the field of fabricating ultra thin, birefringent metal oxide single-crystal films. More particularly, the invention relates to a method for fabricating single-crystal metal oxide wave retarder plates and the use of such wave retarder plates in optical waveguide polarization mode converters.
Wave retarder plates, such as quarter-wave plates and half-wave plates, are components having important uses in manipulating polarization in optical systems. Such components depend on different phase changes that occur between the normal modes of a light wave propagating in anisotropic media rather than on selective refraction or absorption. Referring to FIG. 1, there is shown the index ellipsoid 1000 of an arbitrary anisotropic crystal having principal crystallographic axes X, Y and Z. For a plane wave propagating in the anisotropic crystal in an arbitrary direction defined by a unit vector {right arrow over (u)}, its two normal modes (i.e., the two orthogonal plane polarized components into which the plane wave may be decomposed) propagate at velocites c0/na and c0/nb, respectively, where c0 is the velocity of the plane wave in vacuum, and na and nb are the refractive indices along the minor and major normal mode axes 1001 and 1002 of the index ellipse 1003, respectively. The plane of the index ellipse 1003 is perpendicular to the unit vector {right arrow over (u)}. If nb greater than na and assuming that the two normal modes are in phase as they enter into the anisotropic crystal, a phase difference xcex4xcfx86 between the two normal modes grows as they propagate through the crystal. The phase difference after the plane wave has propagated a distance d through the anisotropic crystal is:                     δφ        =                                                            2                ⁢                π                            λ                        ⁡                          [                                                                    n                    b                                    ⁡                                      (                    λ                    )                                                  -                                                      N                    a                                    ⁡                                      (                    λ                    )                                                              ]                                ⁢                      d            .                                              (        1        )            
The amplitudes of the two normal modes are A cos xcex8 and A sin xcex8, where xcex8 is the angle between the incident plane of polarization and the major axis 1002 of the index ellipse 1003, and A is the amplitude of the incident plane wave. Combining the normal modes after a phase difference xcex4xcfx86 produces a different state of polarization. For xcex4xcfx86=xcfx80/2 the resulting polarization is an ellipse with its major axis parallel to the major axis 1002 of the index ellipse 1003, while for xcex4xcfx86=xcfx80 the polarization is again plane but rotated by an angle of 2xcex8. In the particular case where xcex8=45xc2x0 the ellipse becomes a circle, and circularly polarized light is produced, with the opposite hand of circular polarization obtained when xcex8=135xc2x0. For xcex4xcfx86=xcfx80 and xcex8=45xc2x0, the plane of polarization is rotated by 90xc2x0.
Where the plane wave propagates along one of the principal crystallographic axis X, Y or Z, the normal modes have phase velocities of c0/n2 and c0n3, c0/n, and c0/n3, and c0/n, and c0/n2, respectively. For off-axis propagation (i.e., not along one of the principal crystallographic axes) the two normal modes which are orthogonally polarized relative to one another will be spatially displaced due to the xe2x80x9cwalk-off effect,xe2x80x9d where the two normal modes will be spatially displaced relative to one another after propagating a certain distance. However, for off-axis propagation in a thin anisotropic crystal having a thickness on the order of 10 xcexcm, the spatial displacement, which is less than 0.3 xcexcm, is small relative to the typical beam size used in integrated optics and can therefore be ignored. It is noted that the walk-off effect is not present when light is propagating along one of the three principal crystallographic axes X, Y or Z. Single crystal slabs having planar major surfaces perpendicular to one of the principal crystallographic axes X, Y and Z are referred to as X-cut, Y-cut and Z-cut crystals, respectively.
For a uniaxial birefringent crystal, such as a LiNbO3 crystal, the Z principal crystallographic axis is referred to as the optic axis and n1=n2. The normal modes for a uniaxial crystal are referred to as the ordinary mode and the extraordinary mode, respectively.
Birefringent single crystal slabs that produce phase differences of   δφ  =            π      2        ⁢          (                        2          ⁢          m                +        1            )      
and xcex4xcfx86=xcfx80(2m+1), where m=0, 1, 2, . . . , are known as wave retarder plates, or more specifically quarter-wave plates and half-wave plates, respectively, where m is the order of the wave retarder plate.
Recently, zeroth order half-wave plates have been used in integrated optic circuits to provide TE-TM polarization mode converters. As described in Y. Inoue et al., xe2x80x9cPolarization Mode Converter with Polyimide Half-Wave Plate in Silica-Based Planar Lightwave Circuits,xe2x80x9d IEEE Photon. Technol. Lett., Vol. 6, pp. 626-628, August 1994, an optical waveguide section that supports both TE and TM modes of propagation is provided with a groove perpendicular to the direction of light propagation in the optical waveguide section. A zeroth-order polyimide half-wave plate is inserted in the groove with its optic axis at 45xc2x0 with respect to the electric field vector of the TE and the TM modes in the waveguide section, respectively. Using such an arrangement, a TE wave propagating through the half-wave plate is converted to a TM mode wave, and a TM wave propagating through the half-wave plate is converted to a TE mode wave. The Inoue et al. polarization mode converter uses a 14.5 xcexcm thick zeroth-order half-wave plate made of polyimide and provides low-loss polarization-independent operation at 1550 nm with TE-TM conversion ratios of greater than 20 dB. However, polyimide plates are hygroscopic which adversely impacts the long term performance and stability of the polarization mode converter. In addition, because of its relatively low birefringence, polyimide half-wave plates are relatively thick, which increases the diffraction losses that normally become significant in waveguides with smaller guided mode size, as well as distortion of narrow pulses passing through the polyimide half-wave plate caused by dispersion in that material. Fabricating the half-wave plate using a material having a larger birefringence would permit the wave plate to be thinner, and therefore reduce the diffraction losses and dispersion.
The phase difference, xcex4xcfx86 between the normal modes of a light wave after propagating through a wave plate may be expressed as of xcex4xcfx86=R2xcfx80, where R is the optical retardance. For a half-wave plate R=xc2xd(2m+1), and for a quarter-wave plate R=xc2xc(2m+1), where m=0, 1, 2 . . . . Therefore, the thickness d of a wave retarder plate may be expressed as                     d        =                                            R              ⁢                              xe2x80x83                            ⁢              λ                                      [                                                                    n                    b                                    ⁡                                      (                    λ                    )                                                  -                                                      n                    a                                    ⁡                                      (                    λ                    )                                                              ]                                .                                    (        2        )            
From equation (2) and the definitions of the optical retardance R, it is apparent that the thickness d of the wave retarder plate decreases with its order m and is inversely proportional to the difference between the indices of refraction xcex7b(xcex) and xcex7a(xcex) along the normal mode axes in the wave retarder plate, the difference being indicative of the degree of birefringence of the material of the wave retarder plate in the direction of light wave propagation.
As mentioned above, it is desirable for the thickness d of the wave retarder plate to be as small as possible in order to minimize the diffraction and attenuation losses, and dispersion. Therefore, it is desirable to fabricate wave retarder plates, preferably of zeroth order (m=0), from crystals having a large birefringence, such as birefringent metal oxide crystals including LiNbO3, LiIO3, xcex2-BaB2O4 and LiB3O5. However, forming such ultra thin (e.g., d=10.6 xcexcm for a zeroth-order LiNbO3 half-wave plate at xcex=1550 nm) wave retarder plates of birefringent metal oxide crystals where the wave retarder plate retains the optical properties of the bulk metal oxide crystal presents a heretofore unsolved problem.
In accordance with a first exemplary embodiment of the method of the present invention, there is provided a method for fabricating a wave retarder plate from bulk birefringent metal oxide crystal having a planar major surface oriented with respect to the principal crystallographic axes of the bulk crystal such that a plane light wave propagating in the bulk crystal in a direction perpendicular to the plane major surface has normal modes that travel at velocities c0/nb and c0/na, where c0 is the velocity of the plane light wave in vacuum, nb and na are the indices of refraction along the normal mode axes of the index ellipse, respectively, and nb greater than na. The method comprises implanting ions into the bulk crystal at normal incidence through the planar major surface in a direction perpendicular thereto to form a damage layer at an implantation depth d below the planar major surface. The planar major surface and the damage layer define the wave retarder plate to be xe2x80x9cslicedxe2x80x9d from the bulk crystal. The implantation depth d is given by the relation       d    =                  R        ⁢                  xe2x80x83                ⁢        λ                    [                                            n              b                        ⁡                          (              λ              )                                -                                    n              a                        ⁡                          (              λ              )                                      ]              ,
where R is the optical retardance of the wave retarder plate being formed, xcex is the wavelength of light for which the wave retarder plate is designed to be used. The method further comprises chemically etching the damage layer to effect detachment of the wave retarder plate from the bulk crystal.
According to a second exemplary embodiment of the method of the present invention, the same implanting step used in the first exemplary embodiment is used in the second exemplary embodiment. After the implanting step, the bulk crystal having the damage layer is exposed to a rapid temperature increase to effect detachment of the wave retarder plate from the bulk crystal without chemical etching.
According to a third exemplary embodiment of the method of the present invention, the same implanting step used in the first and second exemplary embodiments is used in the third exemplary embodiment. After the implanting step the bulk crystal having the damage layer is bonded to a substrate, and the bulk crystal having the damage layer is exposed to a rapid temperature increase to effect detachment of the wave retarder plate from the bulk crystal. In one variation of the third exemplary embodiment, the substrate has a first flat and polished planar major surface, and the bonding step comprises placing the planar major surface of the bulk crystal having the damage layer in contact with a first surface of the substrate. The bulk crystal having the damage layer and the substrate are then subjected to moderate heating while using a relatively light force to hold the planar surface of the bulk crystal having the damage layer against the first surface of the substrate for a sufficiently long time to effect bonding of the bulk crystal having the damage layer to the substrate. Thereafter, the bulk crystal having the damage layer is subjected to a rapid temperature increase to effect detachment of the wave retarder plate from the bulk crystal. The substrate having the detached wave retarder plate bonded thereto may be immersed in methanol or water to cause the detached wave retarder plate to be released from the substrate.
In another variation of the third exemplary embodiment the substrate is a borosilicate glass plate having a first flat and polished surface, and the bonding step comprises placing the major planar surface of the bulk crystal having the damage layer in contact with the first surface of the substrate. The bulk crystal and the substrate are then placed between metal electrodes to which pressure is applied to hold the planar major surface of the bulk crystal against the first surface of the substrate while applying a high voltage between the bulk crystal and the substrate to effect releasable anodic bonding between the planar surface of the bulk crystal and the first surface of the substrate. After the bonding, the bulk crystal having the damage layer is subjected to a rapid temperature increase to effect detachment of the wave retarder plate from the bulk crystal. The substrate having the detached wave retarder plate bonded thereto may then be immersed in methanol or water to cause the release of the detached wave retarder plate from the substrate.
According to a fourth exemplary embodiment of the method of the present invention, the same implanting step used in the first, second and third exemplary embodiments is used in the fourth exemplary embodiment. After the implanting step the bulk crystal having the damage layer is subjected to a rapid thermal anneal before chemically etching the damage layer to effect detachment of the wave retarder plate from the bulk crystal. The rapid thermal anneal has the effect of greatly reducing the time required to etch away the damage layer.
In another aspect of the present invention, there is provided an optical waveguide polarization mode converter comprising a section of optical waveguide that supports both TE and TM modes of light wave propagation and that has a groove formed therein perpendicular to the direction of light propagation in the optical waveguide section. A zeroth order LiNbO3 half-wave plate formed by the first, second, third or fourth exemplary embodiment of the method of the present invention is inserted in the groove of the section of optical waveguide. The half-wave plate is oriented such that the normal mode axes thereof are oriented at 45xc2x0 with respect to the direction of the electric field vector of the TE or TM mode of propagation in the section of optical waveguide.